Synthesis of mercaptans



Patented Aug. 3, 1954 SYNTHESIS OF MERCAPTANS Richmond T. Bell, Grays Lake, 111., assignor to The Pure Oil Company, Chicago, 111., a corporation of Ohio No Drawing. Application March 16, 1953, Serial No. 342,710

11 Claims.

This invention relates to the preparation of mercapto organic compounds. More specifically it is concerned with the production of alkyl thioalcohols. This application is a continuationin-part of application Serial Number 148,005, filed March 6, 1950, now abandoned.

The synthesis of alkyl mercaptans may be accomplished by'properly choosing from a number of reactions available which will produce the desired mercaptan in the reaction efiluent. The first reported synthesis of a mercaptan involved the contacting of sodium ethyl sulfate with an aqueous solution of sodium hydrosulfide. Other alkylating agents, such as alkyl halides, will react similarly. Another reaction for the preparation of mercaptans is that in which hydrogen sulfide and olefins are made to react. In such additions of an unsymmetrical addend to an unsymmetrical olefin the predominant direction of addition depends primarily on the polarity of the parts of the addend, other things being equal, but is profoundly affected by a number of other factors, such as the structure of the olefin, kind and location of substituents in the olefin molecule, presence and kind of catalyst, presence and kind of solvent, and any directive agents present either by design or as impurities. With the more common olefins and in the absence of catalytic efiects, a very small degree of addition can be achieved under proper conditions of temperature and pressure, with the addition usually in accordance with Markownikofifs rule. Peroxides, however, influence the direction of reaction besides exerting some catalytic activity. Thus in the presence of peroxides, either added or in the olefin as-impurities as is often the case-predominantly abnormal addition, contrary to Markownikofis rule, takes place and under comparable conditions, the extent of conversion is considerably greater than with no catalyst. Again the presence of metallohydrocarbo-n compounds, such. as the metal alkylsgand aryls of lead, mercury and tin, and'under the influence of actinic radiation having a wavelength below 3000 angstrom units, an abnormal addition with reference to Markownikofis rule takes place. Other processes involve the passage of an alcohol and hydrogen sulfide over a-metallic'oxide catalyst such as thoria. In still other methods mercaptans are formed by a cleavage of disulfides with metallic sodium or potassium sulfide or by the treatment of a Grignard reagent with sulfur. Obviously the choice of reaction is influenced by such considerations as case of operation, reaction conditions, and availability of starting reactants in relation to economic considerations and the particular mercaptan product desired.

The availability of ready sources of inexpensive synthetic alcohols plus an abundant supply of hydrogen sulfide available as a by-product from the petroleum industry makes the condensation reaction in which the hydroxyl group present in an alcohol, such as methanol, is substituted by the monovalent sulfhydryl group from a hydrogen sulfide source in the presence of a metallic oxide catalyst, such as thoria, an attractive and economical process for the production of thiols with the sulfhydryl group in the same position as the hydroxyl group of the parent alcohol and for the production of methanethiol which obviously cannot be derived by addition to an olefin and is difficult to synthesize economically by other methods. In spite of the fact that this method has been academically known for some time for a few alcohols, such as butyl and isopropyl, only a small amount of old, incomplete and relativelly crude data is extant. Relative to the particular use of methanol, such information is substantially inexistent. Although side reactions that take place result in the substantial formation of thioethers, there is scant information in the prior art regarding this substantial conversion ormeans for mitigating the effect of side reactions.

Accordingly, it is an object of this invention to present an improved process for the preparation of methanethiol by reacting methanol and hydrogen sulfide in the presence of a catalyst capable of splitting off water. It is another object to lengthen the life of the catalyst and provide a means for greatly increasing the conversion of the reactants to methanethiol in relation to conversion to other products. Still another object is a method for retarding the formation of undesirable side reactions and thereby improve the selectivity of the process.

My invention consists of the introduction of small amounts of water into the reaction zone wherein methanol vapors are contacted with hy drogen sulfide in the presence of a catalyst to form methanethiol. The water may be introduced into the reaction zone either admixed and/or concomitantly with the reactants, the former method being preferable. I have found that by introducing water into the reaction zone, the resulting yields of mercaptans are greatly increased and that conversely the yield of thioethers is decreased and the catalyst life is greatly lengthened as opposed to the results obtained when no water is introduced.

My invention is further exemplified by the fols fide in the presence of a pumice-supported thoria catalyst containing about 7.3 mol percent of thoria to form methanethiol and dimethyl thio ether conducted at a temperature or 716 F. and a liquid hourly volume space velocity of 8.58, a reactant m'xture containing equimolar portions of methanol and hydrogen sulfide and 0.5 mo percent of water produced a total convcrsi n o 32 percent, based on methanol, ina 4 non run at steady state. Of the methanol that was converted 86 percent was converted into methanethiol which indicates that the oi water mitigates the substantial formation of side products such as the thioethcr. feature, which is termed selectively, is defined as the percent or reacted methanol which was converted into methanethiol. In the absence of water but using the same reaction conditions of time, teinperature and catalyst, the total conversion, based on methanol, amounted to 22 per cent and the aforementioned selectivity decreased to 65 per cent.

When the reaction temperature was increased to 788 F. and the space velocity was varied slightly to 0.59, a reactant mixture containing equimolar amounts of methanol and hydrogen sulfide and 0.5 mol percent water which was contacted with a pumice-supported thoria catalyst containing about 7.3 mol percent or thoria yielded a total conversion of 55 percent, based on methanol, the aforedescribed selectivity being 86 percent. Again the reaction was conducted under the same reaction conditions of time, temperature and catalyst with no water present and a total conversion of 35 percent was obtained with a selectivity of 62 percent. In both of these examples the length of each run at steady state was again four hours.

It is thus seen from the foregoing examples that increases in total conversion of about 58 percent were obtained in each instance when small amounts of water were introduced into the reaction zone.

The beneficial effects of carrying out the reaction between methanol and hydrogen sulfide in the presence of small amounts of water are further demonstrated by the following illustrative experiments:

A quantity of freshly prepared thoria catalyst was divided into two portions, one portion being used to catalyze the subject reaction in the absence of water, the other portion being used to catalyze the subject reaction in the prese- .ce of added amounts of water. The catalysts used in carrying out these runs were in each instance preconditioned according to United States Patent 2,592,646.

In the former instance the reaction was carried out continuously for 95 hours. During this period the reaction temperature was varied between about 705 F. and 750 F., while liquid hourly volume space velocities between about 0.24 to about 1.02 were used. Other reaction conditions which were investigated were pressure, pressures of 0, 5 and 135 pounds per square inch gauge being investigated, and molar ratio of reactants. With regard to this latter variable, ratios of hydrogen sulfide to methanol of $.84 to 4.17 were used. During the investigation of the effect of each specific variable condition investigated, all operating conditions were maintained constant for at least a suficient time to permit a steady state to be obtained for each condition studied. Although substantial conversions oi methanol were initially obtained, the activity of the catalyst diminished to such an extent that at the end of 95 hours of continuous operation substantially no conversion of the methanol was obtained. The average selectivity or percent of reacted methanol which was converted to methanethiol for the entire run was percent.

When, however, the reaction between methanol and hydrogen sulfide was catalyzed with the remaining portion of the catalyst and carri d out in the presence of small amounts of added water, the life of the catalyst Was extensively prolonged and the sensitivity increased.

At the start of the run in which water was used, the ratio of hydrogen sulfide to methanol was 2:1. The reaction was carried out at a temperature of 725 F., a pressure of 139 pounds per square inch gauge and a liquid hourly volume space velocity of 0.35 in the presence of 0.36 mol percent of water. These conditions were n1aintained substantially constant for a period of 205 hours during which time no regeneration of the catalyst was carried out. For this period the average conversion was about i5 percent and the selectivity about percent, resulting in an average yield of methanethiol of about 38 mol percent. Without regenerating the catalyst the run was continued in the presence of increased amounts of water, viz., 3.02 mol percent. After continuing the reaction for about 50 hours under these conditions, the average yield of metl1anethiol for this period decreased to about 23 percent. There was, however, no change in selec tivity. By decreasing the amount of added water to 0.36 mo percent, the yield of methanethiol increased to 38 percent after 9 hous of operating under these conditions. Thus after an elapsed time of 264 hours of operation without regenerating the catalyst, methanethiol yields of 33 percent were being obtained at a selectivity of 85 percent. This, compared with 0 percent yield of methanethiol after hours of continuous operation without using added water, emphatically demonstrates the marked superiority of the instant invention. Prior to shutting down the unit the run was continued for another 12 hours during which period several different pressures and temperatures were employed. During this period the methane-thiol yield and selectivity remained consistently high and the catalyst gave no indication of decreasing in activity.

The unusualness of this invention is further pointed out by analyzing the reaction in light of the laws of chemical equilibria. As one of the incidental products of reaction, water is formed. Therefore one would normally deduce that increasing the molar concentration of water by the addition of more water would suppress to some extent rather than increase the total conversion. Instead, however, a substantial increase in con version. and improvement in selectivity occurs.

Such unexpected and highly desirable results obviously must be caused by effects having no connection with the law of mass action. Also, said effects must be of much greater magnitude in order to completely obliterate any concurrent mass action effect. While I do not intend to be limited in the scope or operation of my invention by any proposed explanation or theory of the mechanism of thephenomenon educed by my invention, I consider that the substantial increase in catalyst activity produced by the presence of small amounts of water is a catalyst surface phenomenon probably arising from various possible equilibria of water molecules or radicals derived therefrom, such as the hydroxyl radical, with the surface structure of the catalyst. With larger amounts of water, the increase in catalytic activity becomes much less in relation to the quantity of water continuously introduced, and in addition decrease in conversion resulting from mass action effect becomes pronounced as the concentration of water in the entering mixture is increased.

In addition to this increase in total conversion it will be noted from the examples that I am able to increase the life of the catalyst and also retard the formation of undesirable side products in the form of the thioethers. Specifically, I am primarilyconcerned with the synthesis of mercaptans and by increasing the conversion ratio of thiols to thioethers, I thus obtain a greater yield of the desired mercaptan.

As indicated above small proportions of water are used. Although the optimum concentration will var to some extent, depending upon the reaction conditions and/or metallic oxide cata lyst used, I find that the described increases in conversion and selectivity are brought about if the water is introduced in an amount between about 0.01 and about mol percent, and it is usually preferable to use water in an amount between about 0.01 and about 1 mol percent.

The reaction of methanol and hydrogen sulfide to form methanethiol may be conducted at a temperature between about 575 F and about 850 F., and at a liquid hourly volume space velocity of between about 0.05 and about 1.5 and preferably between about 0.2 to 0.7. Space velocity as used herein is defined as the liquid volume of methanol passed per hour per unit volume of catalyst. Superatmospheric reaction pressures of about 50-200 pounds per square inch gauge are preferably used, although the reaction can. be carried out at subatmospheric or atmospheric pressures It may be found advantageous to conduct at least the recovery and separation sections of the process under superatmospheric pressures in order to reduce the degree of refrigeration required. For the production of methanethiol, the quantities of reactants employed are preferably substantially in stoichiometric proportion in order to avoid unnecessary complication such as in recovery. However, wide deviations therefrom may be employed and ratios of hydrogen sulfide to methanol between 1:10 to :1 can be used. I

Although the above examples recite the use of a pumice-supported thorium oxide catalyst, a more latitudinous choice of catalysts is available in practicing this invention. The reaction is preferably conducted in the presence of thorium oxide per se or supported-on" apuinice carrier, although other carriers such as activated alumina, bauxtie, montmorillonite type clays, carbon and silica gel may be employed. Although other ratios of carrier to thoria may be used, I have found that a catalyst containing essentially thoria and pumice is sufiicently effective when the composition'of the catalyst has an economical pumice/thoria mole ratio of about 13:1. In determining the mols, the molecular weight of purified pumice was closely estimated to be 67.7, Other metallic oxide catalysts that are effective in carrying out the reaction of my invention are the oxides of zirconium, titanium, uranium, tungsten, molybdenum, chromium, vanadium, manganese, zinc, cadmium, and aluminum. Although all oxides of the foregoing metals are efiective for the purposes of this invention, in cases Where several oxides may be prepared the intermediate oxides between the lowest oxide and the highest oxide, whether well-defined or more in the nature of consistent molar compounds or mixtures, of higher and lower oxides, are preferred.

In the case of zirconium, there is satisfactory evidence only for zirconium dioxide, ZrO2, but if the existence of other oxides were established, they would be higher and lower than ZrOz. Hence Zr02 still would be the preferred oxide of zirconium.

Titanium forms four well-defined oxides, TiO, Ti20s, T102, and Ti03, and titanium dioxide, Ti02, is the preferred oxide with T1202 and the questionable Ti3O4 of lesser degree of preference.

The principal oxides of uranium are U02, U03, and the intermediate oxide, U308, uranium tritaoctoxide, which is preferred together with uraniurn dioxide, U02.

In addition to the clearly defined tungsten hemipentoxide, W205, tungsten dioxide, W02, and tungsten trioxide, W03, six other tungsten oxides, not as well-defined, are known. For example, there is the greenish-blue tungsten tetritatrioxide, W403, the blue tungsten monoxide, W0 (or W20, W02), the blue tungsten hemitrioxide, W203, and the blue tungsten pentitaenneaoxide, W509, and tungsten pentitaoctoxide, W508. This series of blue oxides is intermediate between tungsten monoxide and tungsten dioxide. Another series of blue oxides of tungsten is intermediate between tungsten dioxide and tungsten trioxide. For example, the welldefined bluish-violet W205, the deep purple W303, W401i, and W5014 have been obtained. It is the intermediate oxides, principally those commonly called blue oxides, of tungsten, including W02 which are preferred for the purposes of this invention.

The preference is similar with respect to molybdenum oxides. The existence of molybdenum monoxide, M00, is not certain, but molybdenum hemitrioxide, M0203, and molybdenumtrioxide are well-defined and the intermediate dark blueviolet or violet molybdenum dioxide, M00 2, is fairly well established. Many blue oxides of molybdenum have been obtained with analyses clustered between that of M002 and M003. One commonly called molybdenum. blue is considered to be molybdenum tritaoctoxide, M0308. It has the-unusual property of being soluble in a large number of organic solvents and therefore is of particular interest for the preparation of catalysts to be used in the processes of this invention. A natural molybdenum blue, called ilsemannite, has a composition close to M0308. Molybdenum hemipentoxide, M0205 (or MoOaMoOa), is known as dark violet or violetblack intermediate oxide. Molybdenum-pentita tetradecaoxide, Meson (or 'M0O2'AM003'), is

another intermediate oxide; dark blue', the preparation and existenceof which has been confirmed. Again it is the intermediate oxides, principally those commonly called blue oxides of molybdenum, including M0203 and M002 which are preferred.

In the case of chromium oxides, it is the oxides intermediate between chromium monoxide, CrO, and' chromium trioxide, C1303, which are preferred. Included, for example, are chro mium tritatetroxide, Cr3O4 (or CrO.CrzO3), chromium hemitrioxide, CizOs, chromium dioxide, CIOz (or CI2O4', GT'CIZOS-CI'OB) and a number of less well-defined oxides intermediate be tween ClzOs and C103.

Similarly, with respect to vanadium oxides, it is the oxides intermediate between vanadium monoxide, V202, and vanadium hemipentoxide, V205, which are-preferred; These consist principally of the two well-defined oxidesvanadium hemitrioxide, V203, and vanadium dioxide, V204. However-V205 is onlyslightly less preferred.

The preferred intermediate oxides of manganese are between manganese monoxide, MnO, and manganese hemiheptoxide, M11207, the existence of M1104 being very doubtful and MnzOv entirely out of consideration as a catalyst anyhow. Thep'rincipal intermediate oxides are the well-defined manganese tritatetroxide, M11304 (or MnO.MI12O3) manganese'hemitrioxide M11203 (or Mn'QMnOz), manganese dioxide, MnOz, and manganese trioxide, MnOa.

There-are no' intermediate oxides known for zinc, cadmium, and aluminum, or other establishedoxides besides Z'nO, CdO, and A1203. Thus no preference is possible except that in the case of A1203, it is much preferred to use the modification kn'own'as activated alumina for a carrier and/or catalyst.

While the-foregoing discussion has been directed to the utilization of metallic oxide catalysts for carrying out the instant invention, it is intended that this-invention also may be employed in conjunction with other catalysts which are capable of beneficially affecting th reaction between methanol and hydrogen sulfide to produce m'ethanethiol. Other suitable catalysts include, in addition to'- the aforementioned oxides, the phosphates, halides, sulfates or sulfides of zirconium, titanium, uranium, tungsten, molybdenum, chromium, vanadium, manganese, zinc, cadmium, and aluminum or these metals in their elemental form.

Th mercaptans obtainable by the present invention are useful as intermediates for the production of-methionine and other pharmaceuticals, detergent sulfonium compounds and other surface-active agents, dyestuffs, alkylated hydrocarbon's, compounds important as insecticides and as rubber chemicals, and organic sulfur compounds such as sulfides, sulfoxides, sulfones, sulfinic-acids, sulfonic acids, and thioacetals, and are useful as modifiers in the emulsion polymerization process for synthetic rubber, as solvents or modifiers inthe manufacture of polymerization or condensation products such as resins, plastics, and'elastomers, as anti-oxidants and anti-corrosion agents in'lubricating oil, as antioxidants in stored unsaturated compounds, as assistants in the textile industry, as warning odorants in gases, and in insect-repellents and insecticides.

Therefore, inrecapitulation, it is seen that the essence of this invention resides in the'introduc tion of-small amounts of water into the reaction zonesimultaneo'usl'y with the reactants. I havefoun'd that the addition of water greatly enhances the activity; selectivity, andlength of life of metallicroxide'catalysts employed in the production of thiols' and/or thioethers formed in the reaction between alcohols and hydrogen sulfide.

The expression simultaneously with, used in the instant'cla'ims of this invention with referenceto the introductionof the water into the reaction zone, is 'to be interpreted so as to connote that the said" Water introduction may be effected in admixture with and/or concomitantly' with the said alcohol and hydrogen sulfide mixture; Total charge is to be interpreted as being equal to" the proportions of alcohol and hydrogen sulfide plus'the minor proportions of water introduced into the reaction zone.

It is to'be'understood'that water ineither the liquid or vaponstate maybeemployed and the use of the word-water in the specification and claims is to be construed-accordingly.

I claim:

1. In a process for the production of methanethiol wh'erei'n'methanol is reacted with hydrogen sulfide in the presence of a catalyst capable of splitting off Water at an elevated temperature sufiicient to effect saidproduction, the improvement consisting of adding a small amount of water not exceeding about'5 mol percent, based on total charge,- to the reaction zone simultaneously with the reactants.

2. In aprocess for the'production of methanethiol wherein methanol is reacted with hydrogen sulfide in the presence of a metallic oxide catalyst capable of splitting on water at an elevated temperature sumcient to effect said production, the improvement consisting of adding a small amount of water not exceeding about 5 molpercent, based on total charge, to'the reaction zone simultaneously with the reactants.

3. A process in accordance with claim 2 in which the temperature is from about 575 F. to about 850 F.

4. A process in accordance with claim 2 in which the water is added in an amount between about 0.01 mol percent and about 5 mol percent, based on total charge.

5. A process in accordance with claim 4 in which about 0.5 mol percent of water is added.

6. In a process for theproduction of methanethiol wherein methanol is reacted with hydrogen sulfide in the presence of thoria catalyst at an elevated temperature suflicient to effect said production, the improvement consisting of adding a small amount of water not exceeding about 5 mol percent, based on total charge, to the reaction zone simultaneously with the reactants.

'7. A process in accordance with claim 6 in which the temperature is from about 575 F. to about 850 F.

8. A process in accordance with claim 6 in which the water is added in an amount between about 0.01 mol percent and about 5 mol percent, based on total charge.

9. A process for the production of methanethicl which comprises contacting areaction mixture consisting essentially of methanol, hydrogen sulfide, and Water, said Water being introduced simultaneously with said methanol and hydrogen sulfide in an amount between about 0.01 mol percent and about 5 mol percent, based on total charge, in a reaction zone at a'temperature within the temperature range of about575 F. to

10 about 850 F. with a pumice-supported thoria catalyst, removing the products of said reaction from contact with said catalyst, and separating therefrom the methanethiol.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,116,182 Baur May 3, 1938 2,514,299 Sumerford July 4, 1950 2,514,300 Laughlin July 4, 1950 

1. IN A PROCESS FOR THE PRODUCTION OF METHANETHIOL WHEREIN METHANOL IS REACTED WITH HYDROGEN SULFIDE IN THE PRESENCE OF A CATALYST CAPABLE OF SPLITTING OFF WATER AT AN ELEVATED TEMPERATURE SUFFICIENT TO EFFECT SAID PRODUCTION, THE IMPROVEMENT CONSISTING OF ADDING A SMALL AMOUNT OF WATER NOT EXCEEDING ABOUT 5 MOL PERCENT, BASED ON TOTAL CHARGE, TO THE REACTION ZONE SIMULTANEOUSLY WITH THE REACTANTS. 